What you want is not a collision but a second preimage.
A collision is: find two distinct messages m and m' which hash to the same value.
A second preimage is: given a message m, find a distinct message m' such that both messages hash to the same value.
Second preimages are harder because the attacker does not get to choose both messages at will. MD5 is very broken against collisions (collisions can be generated in less than a second) but not against second preimages. For that, the best known attack is a generic preimage attack which is theoretical only, since it has cost 2123.4, i.e. much higher that what can be done in practice.
So the raw answer to your specific question is: it cannot be done (with existing technology and known attacks on MD5).
(I am here talking only about finding a second preimage on MD5, not about the security of using MD5(salt + data) in any specific context, whether 'salt' and 'data' are known or not. I suspect that anybody who uses MD5(salt + data) actually wants either a password hashing function, or HMAC.)