Hi Roman
ST_Transform is a function to transform a geometry value from one spatial reference system (SRS) into another one. This feature is planned for SPS10, but will not really help in your case.
You are looking for a function like ST_Translate which translates a geometry to a new location by using specific distance parameters. This function is currently not planned but we could implement it.
I already put it on our feature wish list.
If you want to implement the function yourself, you first need to think about your SRS and the required accuracy. The calculation on a planar world is pretty easy, but it's getting more complicated for a sphere or even spheroid. The correct way to implement it would be by using the vicenty's formulae Vincenty's formulae - Wikipedia, the free encyclopedia.
Best regards,
Hinnerk