Hi cwleeming nice to see you in the forums.
As travelislife said you can credit the points to velocity and it may not be worth crediting them to krisflyer instead.
With any loyalty programme it is important to have a long term plan.
If most of your travel is and will continue to be domestic, you are probably better off sticking to velocity and crediting all your points there.
However if you expect to be doing a lot of international travel, it may be worth joining krysflyer.
Of course this also depends on whether you have any current status with velocity, or are making progress towards status.
Singapore Airlines is a member of Star Alliance so krysflyer does generally have more benefits with other airlines than velocity, which has its own network of reciprocal arrangements (including Singapore Airlines & hence krysflyer) but is not part of any major alliance.
I am not an expert on krisflyer but I am sure many forum members would be happy to discuss further if you post some more information about your situation.
One last point - sometimes it can be worth joining a frequent flyer programme even if you don't intend to use it to earn points.
For example Emirates Skyward members may be entitled to free wifi on Emirates flights.
It's worth checking for any benefits like this!
Happy flying!