Clone the pricing procedure and create a new decision table with the Currency field Link the new table to the cloned procedure and swap it into the setup to maintain continuity.

Deactivate the pricing procedure and the associated decision table, then enable multi-currency and add the Currency field to the decision table before reactivating them.

Enable multi-currency in their org, then wait a few minutes for the Currency field to become available for use in the pricing procedure and related decision tables.