i had this problem before too
try this: download apk editor>search for boinc>full edit>files>values>arrays.xml>under name u will find the list of pools u must add grcpool manually and add 'item' tag >save>uninstall the old boinc app then install the modified one using apk editor.
u should find grcpool now in the account manager
let me know if this worked
Just an irrelevant question here, when I looked for BOINC app on playstore, I found that it is related with solar researches . Is it same app you are trying to edit ? And which grcpool are you refering to ?
Update: I feel like it is a marketing idea