hi
I have a similar situation and the response i've got from a UK FTO is that you need to have a UK JAR PPL or CPL to begin the training if you plan to do it in the UK. If it's in the US, I don't think you will need a UK PPL. After that, you must do the MEP VFR course. After passing the VFR skill test you can proceed to the conversion. The requierements to do so in the UK are:
• Minimum 18 years of age
• Theoretical IR (recommended theoretical ATPL)
• ICAO, FAA PPL/CPL SE IR (A)
• JAR PPL/CPL (A) including Night Rating
• JAR MEP
• Current JAR Medical class 1
• Current FAA Medical
• Flight Radiotelephony Operators Licence
• 50 Cross country flight time and 70 total hours PIC in aeroplane or helicopter of with at least 10 shall be in aeroplane.
Course content
• 7 hours in one of our twin engine aircrafts
• 13 hours in Simulator FNPT II
• JAR ME/IR Skill test
The conversion costs around 8000£ and takes more or less 2 weeks.
Another option, is if you'd like to complete the ATPL steps, which is more interesting I guess, is to take the JAA ground school exams, and do some hours on the multi-engine.
Hope this helps !