abap help

hello, i have a question about an issue. i have 2 tables....vbrp and mkpf. i need to take delivery number(VBELN) from vbrp which is defined as CHAR 10 and put it in XBLNR of mkpf which is defined as CHAR 16 but when i write a user exit it says "FOR ALL ENTRIES IN" statement both fields must be of same type and same length. i need to make both fields alike. can someone help? i am pretty new in abap.thanks