The flexible cups also move during pumping to mimic suckling. (thespruce.com)
If you must supplement, do so with an alternative feeding device such as a nursing supplementer, medicine / eye dropper, soft, flexible cup, or a spoon. (motherandchildhealth.com)
If you do purchase bras ahead of time try to find ones that are adjustable and have a flexible cup size. (breastfeedingneeds.com)