JSON-LD import has been implemented in the meantime, so there’s definite pathway for importing existing metadata by converting the Excel/CSV data to a corresponding JSON structure first – this should not be too difficult, but it does require some rudimentary scripting skills.
There’s a similar thread here that might be instructive to read, and we’re also happy to help. To get started, I’d suggest to take one or two sets of your metadata and think about how you would expect the corresponding Tropy items to look after import; then actually create such an item by hand and copy/paste the item from Tropy into a text editor to see the corresponding JSON-LD structure. The exported item will have some boilerplate stuff (specifically, don’t be worried by the whole @context) – in practice you’ll just have to decide how your rows correspond to item/photo metadata and populate the corresponding parts of the item.