Character sheets are HTML and CSS, so if you have a means of converting a spreadsheet or document to HTML, you've got a starting point. That said, only Mentor subscribers can select "Custom" as a campaign's sheet and input the HTMl/CSS manually; everybody else must pick from the published list of sheets. You can still submit a sheet to the GitHub repo to get it added to the published list, but it would be difficult to debug your sheet that way. If you're using an existing sheet, no. There is no means to import the character data from another source.