I agree that it is possible to adversely possess a portion of real property. Pennsylvania statutes provide a mechanism to record with the Recorder of Deeds a written statement as to the adverse posseor's ownership of the property, in case there is a period of time in which you are not in adverse possession, but want to protect your rights. The written statement that is to be recorded is contained in the statute and I can provide you with the law if you would like.
The short answer to your question is yes, you can have adverse possession of just a portion of the real property contained in a deed. The more accurate answer depends on all of the facts, including whether you had permission to maintain the property of another (which would defeat your claim). I suggest you contact an experienced PA Real Estate Attorney to discuss all of the factors before you try to establish your claim to this real property (in other words, do not say or do anything that will defeat your claim until you contact an attorney)..