Add attribute to entity from another entity

i have a microflow that pulls data from mysql and adds it to the mendix database. it also loops over to see if it exists first. i have an entity called credit memos and another called invoices. they have the same attribute but with different names (not ideal but that’s what i have to work with). example: invoices/docID = creditmemos/invoiceNumber. i added an extra attribute in the credit memo entity to hold this invoice number but unsure how to fill in the correct data. does anyone have any idea on how i can achieve this? thanks!