turning a tsvector without position in a weighted tsvector

If I convert a string to a tsvector just casting (::tsvector) I
obtain a vector without positions.
tsvectors without positions don't have weights too.

I haven't found a way to turn a vector without weight/pos, into a
vector with weight/pos.

Is there a way to apply weight/add positions to tsvectors without
positions?
Is there any use-case?

--
Ivan Sergio Borgonovo
http://www.webthatworks.it