For your own account safety we have made it mandatory to do cash transfer to UPI VPA registered in account holder's name only. This prevents cases where a person with bad intent gets hold of your SlicePay account and transfers the money into his own account.
You can always transfer money from your own bank account to another person's bank account using online banking or UPI.
If you don't have a UPI VPA you can read more on how to create one - https://help.slicepay.in/hc/en-us/articles/360009602574-How-to-create-UPI-VPA-