Hi, I'm not familiar with the Aiseesoft app, but Handbrake is an excellent free download used by many on this forum. I would think that any external app that makes the conversion would be fined, with the caveat that iMovie requires Mp4/AAC. AAC stands for Advanced Audio Coding.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Handbrake will convert the video to Mp4/AAC by default. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
-- Rich