You would need to speak to the sales team on 0800 183 1234. As far as I know they would have to cancel the original account and put through a new sale which includes the TV360 box. There are some additional price reductions this week for some of the packages.
I haven't seen any posts from anybody with the Stream (Flex) box being able to pair a third party remote to work with the box, however they can use the VM TVGo app as an additional remote. Providing One Power is enabled on the Stream remote and you have HDMI-CEC enabled on your TV then the VM Stream box should be able turn both devices on and off.
The 360 box only pairs with one VM remote, however there is better news. I have two 360 boxes and can operate them both with my LG TV Smart remote (Samsung smart remotes also work). Instead of choosing Virgin I had to type in Ziggo as the set top provider, and then Ziggo Mediabox Next as thedevice. ( Ziggo is the name used in Holland by Liberty Global (VM's owners))
There are also three One For All remotes that work with the 360.
The OneForAll URC1240, OneForAll URC6820, and the OneForAll URC7125.
The URC1240 and URC7125 are both available from Amazon, and slightly cheaper than from Argos. To set up the OneForAll URC7125 - use the SimpleSet setup process and choose Ziggo as the manufacturer. To set up the OneForAll1240 enter the code 1582, It needs direct line of sight to set it up.
( I haven't tried the URC6820 and don't have a code for that one, but reviews on Amazon say it works)