Q: Is it Okay to use a 61W MacBook Pro charger to charge the ZMI PowerPack 20K?
A: Yes, it is Okay to recharge the battery pack with the 61W MacBook Pro 13" OEM charger (AC adapter). The battery pack will request and get only the power it needs from the charger. All this is done through the handshake and negotiation between the devices in accordance with the USB Power Delivery standard.
By the same token, it is also perfectly fine to recharge the battery pack with the 29W/30W MacBook 12" charger or the 87W MacBook Pro 15" charging block.