Summon Evil/Good Monster feats give you more options and the dretch is on the evil list.
A Dretch can be summoned using Summon Monster 3 - no special steps are needed.
Alternatively you can use the Planar Binding and Planar Ally spells, which have their own rules.
There are a few ways to summon creatures: The Summon monster spells, the Summon nature's ally spells, various Planar ally and Planar binding spells, and Gate. These lists can be improved by the different rings of summoning affinity, each of which adds a group of creatures such as divs, demons, archons, and so on, the Summon Good/Neutral/Evil monster feats, and probably stuff I missed.
The issue is that summoning is really limited to these lists. Anything outside them is no-go, mostly I think for balance reasons. That said, it would be quite possible to REPLACE those lists by others, say, by letting a player summon from a different list, but barring him from the normal Summon Monster spells.
There are some extra options as cleric and those new rings from ACG. They offer a few more options too.