You can attach or link time entries to existing invoices, draft or final, after the billing process is over by using the Progress Billing feature in CORE. After you approve the time entries, you should be able to add them to your existing invoices, a process called releasing time entries (or progress billing).
If you have already processed and sent invoices to the clients, and then received the approved time entries, you can associate those entries to the previously processed invoices at a later date. You can do so from the Batch Update option in Time Entries by selecting those entries and linking them to that invoice. Note that this option is only visible when you are viewing the time entries by project. So make sure to select Project in the Show drop-down of the screen before using Batch Update. CORE automatically recalculates and adjusts the difference in the value of time entries and invoice amount by generating a write-up/down. Note that this won't change the invoice amount. Check CORE Help Center for details on releasing time entries.
You can do the same to expense entries.
Note: You should attach time entries in this manner only when you are pre-billing a fixed project. If it is an hourly invoice, you should rather delete the draft and recreate the invoice.