Fixtures, HABTM and id values

I have models Frog and Froglet with HABTM relationship between them.
In the fixtures file for froglets I can say
frogs: frog1, frog2, frog3
where frog1 etc are the names in the frog fixture, and the join table is magically created for me.
This works fine provided the fixtures do not explicitly include id values for the frog and froglet records.

Unfortunately I have legacy code that requires that I explicitly specify the ids in the frog and froglet fixtures. Is there anyway that I can specify something like:
frogs: 1, 2, 3
where 1, 2, 3 are the id values obviously. I tried this but the join table was not correct.

Colin