Adding multiple characters by a single keypress in X11 with XKB.

[An-Najmus Saqib]

Dear Xorg Volunteers,

I use a third-party software for Bengali typing in Windows named Avro
Keyboard. It provides a graphical keyboard layout editor (only in
windows-version, linux version supports only phonetic bengali typing).
I made my own layout with it as well as I'm using it for several
years. And I can type multiple Unicode characters with it (for Bengali
conjuncts).

A few months ago I started using Ubuntu (12.04). One of the biggest
problems I faced it was my Bengali typing. Then I started googling and
found here a way to make xkb layout in Ubuntu. Now I am writing a new
bengali custom keyboard layout in xkb. In Bengali I need to add some
conjuncts in my layout which is made up by multiple unicode
characters. How can I type those characters in a single keystroke? In
my layout I need these characters :

   key <AD04> { [   U09B0,  U09CD_U09B0,  U098B   ] }; // BENGALI RA,
BENGALI RO-FOLA, BENGALI VOCALIC R
   key <AB01> { [   U09AF,  U09CD_U09AF           ] }; // BENGALI YA,
BENGALI YA-FOLA
   key <AB02> { [   U09B7,  U0995_U09CD_U09B7     ] }; // BENGALI SSA,
BENGALI KHINYA

I searched the Internet, most of answers were about xim by editing the
compose file; it looked like a per-use setting. But I to use it
systemwide by xkb.

And  my Arabic layout because UFEFB is not supported (in search
option), so I need to use U0644_U0627.

Here I got that xkbcommon supports a syntax like this (but it's not
supported in X11) :

       key <HELO> { [ h, i, { h, e, l, l, o }, { H, E, L, L, O } ] };
// to produce 'h' from level 1, 'i' from level 2, 'hello' from level
3, and 'HELLO' from level 4.

Is there any alternative syntax for this in X11?