Depends on your definition of "most."
On the one hand, you would be making your home more functional--and a 3 bed/2 bath is far more desirable than a 2/1.
On the other hand, your home will always have the same square footage. And whether you can legally call it a 3 bedroom/2 bath depends on size of the rooms, size of emergency exit, presence of closets, and so on.
If I had to guess--and it's only a guess--the value of the property probably would be between that of a 2/1 and a 3/2. For example, if 2/1s are selling for $180,000 and 3/2s are selling for $230,000, then yours might end up being worth about $205,000-$210,000. Now, if my numbers are correct and you could do the conversion for $20,000 or so, then you'd recover your expenses. But what you really need is for a Realtor familiar with the area to come in and give you solid numbers on both 2/1s and 3/2s, as well as an educated guess about what adding a bedroom and bath to your townhouse would do for its value.
Hope that helps.