String operations for DMEE element

Hello experts!

I'm trying to implement one simple condition for the DMEE tree element (t-code: dmee)...

I faced some difficulties with the list of predefined functions as they don't allow to do what I need.

Let's say I need to do some sort of string operations under REGUH/VBLNR field (document number) and current value on this field is 2000012050.

Second step, I need to strip off the first digit from this number, ie it'll be 000012050

Third step is to overwrite next 4 characters with zeroes (in case they are not zeroes), ie final number should be 9 characters long with four leading zeroes: 000012050.

Fifth and final step is to write the result to the DMEE tree element:

As you may see, I put structure field in place... In the output xml file it will show document number, but without those string operations I shown above. I noticed another one field on 'Attributes' tab, ie 'Conv. functions' (I suppose it means 'conversion functions'). There are lots of them and none of them is suitable for me. Moreover they dont have proper description:

So, I'm a bit lost... how can I apply conversion logic (string operations) to this field, shown above? Is there any other ways to apply this logic rather than to write an 'exit function'???