You shouldn't (different formulas / detergent mixes, blah blah blah)
But you can add if it's a top up before a change, and it's the same body.
If it's a change and would like to switch to something else, you can, but for reasons above I'd do two changes over a shorter period. say 3000km instead of the of the 5000.
At least that's I remember from Sanctifier's - The truth about oil...................
Take a read through and verify.
Which is it switch or topup?