There is no conversion for CPL as such, you just do the normal CPL exams (written test, oral exam and checkride). Provided you fulfil all FAA hours requirements, you just need to do a couple of hours for preparation with a CFI. If you already have a CPL, then TSA clearance is not required (it is for IR however).
Getting a "based on" license is possible for a PPL/IR, this is just a paperwork exchange with the FAA without doing any flight tests (see Part 61.75). But keep in mind you are restricted to whatever you are allowed to do on your base license, i.e. if you don't have a valid R22 TR, then you cannot log PIC. Also, the license verification letter is usually only valid for 6 months, so you'd have to re-do the paperwork every time you go to the States.