Could you change the current /lp user parent addtemp to literally to add a duration to the current duration and add a new command /lp user parent settemp to set the duration.

This may already be done here: https://github.com/lucko/LuckPerms/wiki/Configuration#temporary-add-behaviour

The parent settemp isn't a thing yet, so that'll require luck's aid. But I hope that link shows how to add a duration to an existing temporary group/permission/meta erg.

That was supposed to be etc* at the end. Whoops. Damn autocorrect got me!