How can I import my Zoho Projects timesheets into Zoho Invoice ?

Hi ! I use Zoho Projects for years and now, I'm testing Zoho Invoice in order to see if it could replace my current billing system. I setup the whole thing and I'm ready to send my first bills produced by Zoho Invoice. The only problem is that I can't see how I can "import" / see / use my Zoho Timesheets for billing. As all products seems to be related with the token, why can't I have access to my timesheet from Zoho Invoice. I don't want to dupplicate all entries. Thank you !