This is my ~/.Xmodmap. It adds German umlauts to the default (no deadkeys) US layout.
!! umlauts on a US keyboard !! keycode <number> = <simple press> <with Shift> <with Mode_switch> <with Mode_swtich and Shift> !! <Mode_switch> on a european 105-key-layout is usually AltGr keycode 26 = e E EuroSign EuroSign keycode 38 = a A adiaeresis Adiaeresis keycode 30 = u U udiaeresis Udiaeresis keycode 32 = o O odiaeresis Odiaeresis keycode 39 = s S ssharp keycode 108 = Mode_switch clear Lock
Download here.