if you try to put draft on, let say iges import, you will have 90% failure.
For make your draft design, you cam split that part in 2-3 mother surfaces: first mother which is not affected by draft/pull direction, second for +draft, third surf for -draft.
For this you can use Pro/Legancy. For surfaces wich will be drafted I suggest to add material (try to use one offset parting line to have one solid edge, after that use boundary surfaces - if is complex shape). Sometime it's work well copy surface (offset with draft delta, then drafted after curve, after that you can choose tweak->replace)
this are some tips.
by-the-way I'm not a guru ... , only sometime one Pro/E user!
I would suggest you remodel it. This would probably take less time in the long run the messing around with an imported part. When you start trying to change imported parts, you will run into all kinds of trouble.
Remodeling will give you a good model to work with and save you much frustration.
To help with your remodel, go ahead and take your imported part and make a temporary drawing by throwing on the dimensions you will need to remodel. Then when finished with your remodel, assemble your new one over the old to see that everything matches (although it really won't match since you are adding draft, so never mind).