local xkb layouts

[Stefan Witzel]

Hello,

is it possible to extend the system-wide xkb layout database by 
user-specific files. And if not: why is that?
I ran into this problem because I want a german 
"deadgraveacutecircum"-layout in which grave acute and circumflex are 
dead but tilde is not. But on systems where I am not administrator I 
cannot install this. And even on systems where I am administrator I 
would have to renew my changes after every upgrade.
Would the "right" way to do it be to use the standard layout (with dead 
tilde) and then use a .Xmodmap? Every time I tried that I got strange 
effects when switching between multiple xkb layouts.
Thanks in advance for any help!

Best,
Stefan