As a Completesubscriber You have different functions to take branches from other trees. Smart Matches, Record Matches, and Instant Discoveries.
If You need more from a fellow tree, write to this site manager and ask him for the tree data, or part of it as a GEDCOM file.
With the Family Tree Builder (aka FTB) You are able to merge this GEDCOM files into Your FTB project.
The risk of all these options is: You could get inconsistent, bad verified data into Your tree.
For instance, I don't use these functions to extend our family tree automatic or half-automatic. I take the data this functions did found as possible correct data, verify every one, and put it, if it's ok from my point of view, manually into our tree. The advantage of this hard way is to have a most clean, curate tree.