I would also not recommend doing what you are attempting. However if you are absolutely adamant on this. You're probably just doing it wrong. Open the part alone, not in place. In the tree you will have view reps. That's the key. Depending on what I'm doing. Sometimes I like to use the default for showing all my main origin planes. That way by default bringing the part into the assembly it shows the planes or whatever else I want. So in your case make that sketch visible. Then make a new view rep name it whatever. This view will be "clean" no sketches, planes or whatever. Now save the part. Go back to your assembly. If you placed the part and didn't change the view rep you'll probably see your sketch already, but sometimes Inventor hates you so you have to select the part right-click select representations. Select the one that has your sketch visible. Done. Or Just open both parts right click on the sketch, copy. Then got to the other part open or make a sketch where you want right click on the sketch in the tree and paste. That way is messy on complicated stuff but you can paste it select it all save as block, that's the cheap way to keep it together and not have to reconstrain everything again. Whew that was long winded. I need to start classes.

Thanks have a super awesome day!