how to add primary key to existing table with multiple primary keys

<div class="Section1"><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Hi there,</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">I am trying to add another primary key to an existing table with two other primary keys.  I got the
followingerror when I tried this command:</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">alter table extprobe2tissue ADD primary key (expid);</span></font><p class="MsoNormal"><font
face="Arial"size="2"><span style="font-size:10.0pt; 
font-family:Arial">ERROR:  multiple primary keys for table "extprobe2tissue" are not allowed</span></font><p
class="MsoNormal"><fontface="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">The details of the extprobe2tissue table is: </span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">\d extprobe2tissue;</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">          Table "public.extprobe2tissue"</span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial">   Column    |          Type          | Modifiers</span></font><p class="MsoNormal"><font
face="Arial"size="2"><span style="font-size:10.0pt; 
font-family:Arial">-------------+------------------------+-----------</span></font><p class="MsoNormal"><font
face="Arial"size="2"><span style="font-size:10.0pt; 
font-family:Arial"> probeid     | integer                | not null</span></font><p class="MsoNormal"><font
face="Arial"size="2"><span style="font-size:10.0pt; 
font-family:Arial"> tissueid    | integer                | not null</span></font><p class="MsoNormal"><font
face="Arial"size="2"><span style="font-size:10.0pt; 
font-family:Arial"> value       | double precision       | not null</span></font><p class="MsoNormal"><font
face="Arial"size="2"><span style="font-size:10.0pt; 
font-family:Arial"> entrydate   | date                   | not null</span></font><p class="MsoNormal"><font
face="Arial"size="2"><span style="font-size:10.0pt; 
font-family:Arial"> description | character varying(200) |</span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial"> qval        | double precision       |</span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial"> expid       | integer                | not null</span></font><p class="MsoNormal"><font
face="Arial"size="2"><span style="font-size:10.0pt; 
font-family:Arial">Indexes:</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">    "extprobe2tissue_pkey" PRIMARY KEY, btree (probeid, tissueid)</span></font><p
class="MsoNormal"><fontface="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial">    "extprobe2tissue_probeid" btree (probeid)</span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial">    "extprobe2tissue_tissueid" btree (tissueid)</span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial">Foreign-key constraints:</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">    "extprobe2tissue_tissueid_fkey" FOREIGN KEY (tissueid) REFERENCES
tissue(tissueid)</span></font><pclass="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial">    "extprobe2tissue_probeid_fkey" FOREIGN KEY (probeid) REFERENCES
extprobe(probeid)</span></font><pclass="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Any suggestion would be much appreciated.</span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Thanks!  </span></font></div>