Yes, that is, of course, possible.
Your Lifesum account saves all your data regardless of a device you’re using. If you started e.g. using Lifesum on your phone and then decide to use it on your tablet as well, you just have to make sure that you log in with exactly the same login details on the tablet.
You will then have access to all your Lifesum data you’ve entered on your phone so far. Everything you enter on your tablet will then automatically be synchronized to your phone again.
The data will be synced within minutes.