We know that "can" has the same meaning as "is/are able to".
(In the future) I will add more seats so that more people will be able to sit down.
(In the future) I will add more seats so that more people can (are able to) sit down then.
For the second example, is the temporal marker "then" required for "can sit down" to refer to the future?
PS. There's a mistake in my subject headline. It should be "to refer to the future" and not "to the future".
I think the 'then' would be helpful, as you can't mark 'can' for the future (at least, not in standard English - I think there are some dialects (Geordie?) that accept 'will can').
But adding 'then' at the end to clarify the future sense of 'can' doesn't sound fluent, to my ear. I think most native speakers would use a to-infinitive: '...to let more people/allow more people to sit down' or even something like 'to accommodate a bigger audience'.