Currently, besides setting your Default FTP, there is no way to manually enter an FTP number. This functionality is on our development list.
One way to “trick” the system would be to do a 10min “super effort” before or just after a hard near-FTP effort. This would likely give you a number very close to your actual 20min best. Then, you could simply mark the workout as an FTP Check and VeloPro do the rest.