Effects cannot extend footage. This has never been any different. You are mistaken. Only the native time-remapping can do this. The rest is unclear since your descriptions are rather vague and lack any genuine technical info. Otherwise AE doesn't discriminate - a layer is a layer is a layer and whether that's a DynamicLink element or a natively imported video doesn't matter.
Thanks for your answer Mylenium, I appreciate it, if you could help that would be amazing! I never used native time-remapping before so now I'm confused. So to be clearer, here is what I'm trying to do:
Drag a 10 second clip directly into AE, or dynamically link a 10 second clip that's already in Premiere to AE by right clicking on the clip in Premiere and selecting "replace with AE composition." Since I was filming the clip knowing I'd do for slow motion, it is 60fps and a high shutter speed.
I would like to slow this clip down to 50% in AE with time warp and not lose any of the clip. So, theoretically, a 10 second clip slowed down to 50% should come out to be 20 seconds.
Please help. Any help is appreciated.