This question has been raised in many forums.
Mod teams everywhere want to know if they can import their work.
The response seems to be yes, with conversion as indicated by Jobabob....
But before you get too excited, dig out your first playable map (one that actually works) and use all the knowledge you have gained to improve it....
I bet you won't be mapping for long before you decide to scrap the map and use the original concept to start again.
So, what makes you think that transfering to HL2 will be any different? Most of the stuff you have done so far will be unworkable/unusable/look incredibly stupid in the new engine.
If you want to take the brush work with you, fine.... but you might want to spend a month working out how the new editor works before you do that.