extra chars and mask files
Given this set-up to include some extra German characters:

./oclHashcat64.bin -m 7900 -1 äöüß?l -2 ÜÄÖ?u -3 €?s -a 3 --increment hashes.txt rockyou-7-2592000-copy.hcmask

I could simply use one of the rockyou mask files by making a copy and doing a simple find & replace to switch out:
1 for l
2 for u
3 for s

Correct ?
That will work indeed. Note that in this case you're cracking ISO-8859-15, so make sure not use use UTF-8 in your editor.
Hi Atom, thanks for your help.
I think that the character encoding is the issue..

I've changed my terminal encoding ( export LANG="ISO-8859-15" )
and even attempted to change the hash files encoding ( iconv -f UTF-8 -t ISO-8859-15 um.txt ->um15.txt )

but still no joy with the following command to catch a hash that should resovle to 'Äü':
./oclHashcat64.bin -m 7900 -1 üäöß?l -2 ÄÖÜ?u -3 €?s -a 3 um15.txt ?2?1

This also fails to catch it: ./oclHashcat64.bin -m 7900 -1 üäöß?l -2 ÄÖÜ?u -3 €?s -a 3 um15.txt Äü

Any encoding will still catch simple passwords with all digits, or all lowercase: ?d?d?d, ?1?1?1

I'm pretty new to this and fear I'm doing something boneheaded here..

If it's important, the site in question was origanally built on an American server, then transfered over to a German one. Passwords were created on each instance with German characters. And I can login no problem with the 'Äü' password.

If it helps at all I can post the hash.?
The problem is, if the hash was calculated while the encoding was set to UTF-8, you must attack it using UTF-8, too. Attacking masks with UTF-8 is a bit more complicated, see this blog post how to do it: