Yes, of course you can...but bear in mind that it won`t sound quite `right` and you won`t get that full viola flavour.
Give it a try...and be prepared to put your violin back as a violin if it doesn`t sound `right` to you...
Another option that I suggest is to first rent a viola for a month (so the expense doesn`t get too high)...get a feel of how it plays and sounds...and then try switching your violin over...and you might find yourself making do with your converted violin more readily.
What I`ve found most noticeable in playing viola is the size of the instrument...it`s much bigger and therefore harder to hold (and uncomfortable). You also have to stretch your fingers much more to reach the proper notes...you won`t experience this on your convertd violin at all. But if you rent the viola first...you`ll at least appreciate this major difference...
The third item is the alto clef...that`s what I`m having the most problems with...I know where all the notes are...but I still can`t play to speed...or if I try I find myself playing the alto clef as if it were the treble, which of course, doesn`t help matters any.
Good luck! Let us know what you do!