Yes, but typically, the reasons involve either the evil character having a code of honor (Lawful Evil, similar to Canderous from Kotor) or there being a bigger threat, that causes a temporary alliance (Wrex gives a similar reasoning for this joining you during the first Mass Effect.)
Sure, there are other possible reasons - the character claims to owe their life to a party member (Hanharr, from TSL or Morinth from ME2, though that probably falls under the whole "code of honor" thing), or they're a servant/minion of, or have some other close relation to a partymember (HK-47), or they're after some sort of powerful item which/title which means you can trust them for a certain amount of time (I'll admit, I can't think of an example for this one. I'm sure there's one. I'm just drawing a blank. 0o)
Point is the evil character, in theory, can have whatever reason they want for helping out the party of do-gooders. However, there needs to be a reason as to why the party trusts them enough to tag along. That is a little more complicated. It has, however, been done in Bioware games. HK-47 might not be Masquerade: The Bloodlines level evil, but he's an assassin who takes a great deal of pleasure in executing his targets. And he's very clearly marked as "dark side", as is Canderous.
However, all of this is rather moot anyway, when discussing Dragon Age - as the only definite "evil" we have in Thedas are the Archdemons - everything else is rather shades of grey.