It´s not possible to open parts for modeling in PSonTC. The locked status is correct.
You need to modify your part e.g. in NX also connected to TC.
Do your parts have 3D representation (based on JT dataset)?
I think you cannot select it as part in the dialog because of missing 3D representation.
I do slightly disagree at this point. May be it has been changed meanhwile but in the past (at least til version 12) is was possbile to set part to modelling scope and modify geometry or kinematics. Doing this the changes could not be stored (end modelling) to the part directly but user was able to create an additional Direct Model dataset on selected operation.
I assume this should cover the use case when you have to do changes on the part from simulation perspective.
Not to mention that this is ending up in an issue with visualization in TC MPP Viewer when displaying the operation that has the part consumed -> you'll see both the original one and the modified one that is attached to the operation itself.