We found a workaround for ourself in this case:
Define an adaxes-internal String attribute (for e.g. adm-CustomAttributeText10)
Before sending the approval, upadate the User who will get the approval an write your text to the attribute mentioned above.
Upate the approval and add the attribute beyond the default text.
After sending the approval clear the attribute again.
It will look like this:

Not ideal but workable...
@Adaxes-Team: It would really be helpfull do have the possibility to do this natively!