XCompose: How to map a combination composed by a "basic" character & a character defined in a private use area?

[Pierre-Luc Angles]

Dear xorgers,

I have already mapped without problem characters defined in a private
use area in my ~/.XCompose file like this:

<dead_psili> <dead_grave> <Greek_UPSILON>                  	: ""	Ue1ad

But now I have problems by mapping a non-spacing mark (or diacritic)
combined with a basic character like the Greek iota. I have tried to map
it like this without success:

<dead_psili> <dead_diaeresis> <Greek_iota>
      : "ι"

Could anyone please tell me how it is possible to solve?

Thanks a lot in advance!

Pierre-Luc