No, you cannot add more funds into an existing Planned-Spend Account at this time. We are working on this product enhancement and hope to be able to offer Relay customers this flexibility in the future. You can, however, load additional funds onto your Relay Card. Note that you will not earn cashback rewards on any additional funds that you load onto your Relay Card. Your cashback rate only applies to your monthly payouts generated from your Planned-Spend Account.
See this FAQ for more information on how to load additional funds to your Relay Card.