If the verb in the subjunctive needs to be in the past tense, then the subjunctive past tense must be used. It's completely different from the present subjunctive.
To form the subjunctive past, start with the third person plural (or ustedes form) of the imperfect tense, then remove the -ron. After that, add the proper ending to the stem to complete the tense:
|usted, él, ella||-ra|
|ustedes, ellos, ellas||-ran|
+ Notice that this conjugation also gets an accent on the final part of the stem so that stress is consistent: fuéramos