Hello,
Sage X3 has data import models for common items. Your data must be structured before or during this data transfer.
You have to be careful not to take too much data, which would pollute the new system. You have to be aware that it is a technical work, time consuming.
Getting started with your date and your (limited) history also allows you to validate that the new system allows you to work at least as before.

Yes it is possible but the real question is about the effort you are willing to spent. You can use standard template to import past sales (prerequisite to import beforehand all past products and past customers) and then you need to import the remaining step of the flow: delivery (and related stock), invoices, payments, credit notes, return. To keep the consistency of what really happened, you will need to import lots of documents. You will load a new solution with lot's of unnecessary information for the current business. To me, it's like when moving in a new flat: it is the great opportunity to sort all my stuff and get rid of what is unnecessary.

So what benefits are you looking for?

- If this is for reporting, then I suggest to use a BI tool and have a datamart or a cube feed by both systems (your legacy and Sage X3), this will be cheaper and you will gain a powerful tool in the process

- If this is for other reason, a simple approach is to load the data in custom table mirroring the structure of the standard ones (one of the key benefit of Sage X3 is how easy you can customize the solution). In that case, you can have only the past sales without bothering to migrate all the related documents. If you expect the solution to process such data (e.g. forecast calculation), then you will need to push the customization 1 step further and develop some code.

complete agree with your approach