Of course it is. In fact, this mod in particular is becoming well known for one reason

The creators of the mod somehow managed to track down the original voice for Krystal and got her to re-record Alyx's lines.

In the case here the VA was apparently an associate of the mod developer who had been communicating with him for some time. Additionally, a lot of Rare VAs at the time were just other employees they had around at the time with Krystal's VA being a translator or something so they weren't bound to usual VA contracts that would obstruct this sort of thing. It's a weird set of circumstances that just seemed to allow for this sort of thing.

Though why they wanted to put her in Half Life 2 of all games I have no earthly idea.