Hi. Nothing I am afraid. There is no way to pair Roam and Sub. This is a deliberate design choice by Sonos. 

Thank for quick reply.

Even if I wait in the future
Is it useless expectation?

I would expect so. The Roam lacks the antenna/electronics necessary to have a ‘bond’ such as you would do with a Sub.

I don't think it would be a great sounding combination anyway, although I don't think that's the reason Sonos doesn't allow it. (A pair of Moves plus Sub would probably sound very good but is also not possible. )

But I agree with Bruce that we are unlikely ever to be able to put this to the test. 

I think you’d do much better by adding a second Roam and thusly having a srereo pair!

Less money and you’d have a killer stereo set up…..The Roam can go pretty darn low.I believe it has usable output down to 60 hz.

A pair will play louder and it makes for a killer little stereo system.

Good luck

The OP already has a pair of Roams, and wants to add a Sub to that pair.

Everyone’s needs are different, but if the key aim is a stereo pair at home, then a One/One SL pair would give significantly better sound at a fairly small extra cost.  And it really sounds great with a Sub.  Of course, the Roam has many other capabilities that some users will be able to exploit.