Depending on the cavity you are filling (and the viscosity of your injected media) you may need to create channels to the exterior of the part to evacuate air. You can always back fill these with clear resin later and cure to make them nearly impossible to see. Just something to consider. If your injection site is large enough it might be a moot point.
I’m fairly certain Formlabs has written a blog entry on this exact process, search around and see if you can find it.