Welcome to the ImageJ forum, @Seb!
You can always use
Image > Stacks > Images to Stack to fuse single images into a stack (or Image > Stacks > Tools > Concatenate... to fuse several stacks), and then:
Image > Hyperstacks > Stack to Hyperstack to define the number of channels and slices
Note that the Composite display mode (e.g. from Image > Color > Channels Tool...) won't work when working with more than seven channels, but Color mode works, and you can define custom LUTs per channel.