Hi Jurgen,
This sure comes at the right time. I was investigating method to extend Material master for certain client too.
So if I understand this properly:
Option 1
OSS note 78093 - Display of alternative material identifiers
This method can be applied for the standard ERP by using the MARA-BISMT field and using those conversion exit as specified in the notes. My understanding is that, you just have to maintain conversion exit and index for MARA-BISMT, and for example, you create a PO through ME21N, you can enter a record exceeding 18 character (though the note also says some column display length will not automatically stretch above 18 chars and thus will require scrolling.)
I have also searched for changes to MARA-BISMT and some users mention that this field cannot be changed exceeding 18 char (thus invalidating the note above?)
Option 2
1597790 - Activation of Long Material Number in DIMP
This is from your other post, Length of MARA-BISMT 'old material number' | SCN
I cant test this, but this should be a straight forward configuration available only to Industry solution installations?
Anyway, thanks for the post it provided a lot of info to go with.