The only way to do this is to screenshot an existing TripIt itinerary and use Trip.com's image import functionality.

How well does this work?

Very well in my experience, however, there is a bug on Trip.com's side on the calendar sync.

Trip.com Bug

While the imported flight time shows up currently in the itinerary view, when I've booked a flight departing at 12pm (noon) on the 15th, the calendar syncs that flight to the 14th, the day before.

Why use Trip.com?

I think it's the best OTA out there. You can read about it in my review of the Trip.com loyalty program.

Can Trip.com Travel Log replace TripIt?

No, the import method above only works for small amounts of data, and Trip.com is missing quite a few features that are sticky for TripIt users.

What about bulk importing into TripIt?

The short answer is no, you cannot do it today. There are 2 parts to solve for this question.

1, Can you export data from TripIt?

The answer is yes, but it's not user-friendly. You can go down one route by using GitHub scripts. If you scratched your head at the word GitHub, that option is not for you.

What I suggest instead is you email TripIt support and make a GDPR full data dump request, which will get you a dump in JSON format.

2, Can you import the data into Trip.com?

Trip.com would need a JSON import feature.