Possibly better to wait until you have the ATPL though there is no real benefit...
IF you have an ICAO ATPL and a valid TR then you can simply write the 14 ATPL theory exams then do a check-ride on type for the conversion.
If you don’t meet those criteria then you have to do the 650hours study requirement and write the exams. Once completed you can then do the CPL conversion in any aircraft that meets the requirements for CPL issue - the CPL course is 25 hours and you’d be exempt from some of that - exactly how much depends upon when you do it and which National authority you work through.
Typically max credit would be 18 hours leaving 7 to get familiar with type/airspace/local regs etc.