Hello Malcolm
Currently you can not import in your Desktop invoice template.You can import a word(doc x file) into the QuickBooks Online as an invoice template..If you go to the cog in the top right of QuickBooks and then go to QuickBooks Labs and then toggle on the import style option and then hit on done
If you then go to the cog and go to custom form styles and in the top right where it says new style and there will be an option for import style.If you click on that and then click on download a sample.It will then give you a how to guide as well as two examples which you can use as guiding templates
Thanks
Emma