Below is an excerpt of 14 CFR Part 61.57 concerning recency of Instrument experience; it does NOT state that this needs to be done with an FAA certificated instructor or on a N-reg airplane.
Which means that all of the approaches you practice with a JAA Instructor in preparation for your IR count towards your currency.
So your FAA IR is "current" or "valid" for your JAA IR conversion.
If your FAA IR is less then 24 months old that counts as the IPC/BFR
(c) Instrument experience. Except as provided in paragraph (e) of this section, a person may act as pilot in command under IFR or weather conditions less than the minimums prescribed for VFR only if:
(1) Use of an airplane, powered-lift, helicopter, or airship for maintaining instrument experience. Within the 6 calendar months preceding the month of the flight, that person performed and logged at least the following tasks and iterations in an airplane, powered-lift, helicopter, or airship, as appropriate, for the instrument rating privileges to be maintained in actual weather conditions, or under simulated conditions using a view-limiting device that involves having performed the following—
(i) Six instrument approaches.
(ii) Holding procedures and tasks.
(iii) Intercepting and tracking courses through the use of navigational electronic systems.
(2) Use of a flight simulator or flight training device for maintaining instrument experience. Within the 6 calendar months preceding the month of the flight, that person performed and logged at least the following tasks and iterations in a flight simulator or flight training device, provided the flight simulator or flight training device represents the category of aircraft for the instrument rating privileges to be maintained and involves having performed the following—
(i) Six instrument approaches.
(ii) Holding procedures and tasks.
(iii) Intercepting and tracking courses through the use of navigational electronic systems.