Technically it is possible. But it will require a lot of effort on your part with lots of steps.
Ambient Weather gives access to the data via their published API
https://www.ambientweather.com/api.htmlYou could write a program to extract the data using their API.
Then you could import that data to weeWx and then use the wunderfixer extension (an add-on to weeWx) to publish the missing data to Weather Underground.
http://www.weewx.com/wunderfixer/Maybe you could enlist the help of a software developer if you yourself are not a programmer. Maybe someone has already written a piece of code to extract from Ambient Weather Cloud using their API to save into a format that can be read by weeWx. I've not yet come across such a piece of software yet but it doesn't mean it doesn't exist. Realize that Ambient Weather Cloud came out not long ago and it hasn't even been a year since they published their API.
Even after you have successfully uploaded the missing data to Weather Underground there may be some quirks. Read the following thread where Tom the programmer of wunderfixer has some interesting insight to uploading data from before you actually joined WU. The data will be there but maybe not in the way you expect. This part isn't totally clear but maybe you can have a read yourself to see what I'm talking about. But even if you can't easily see the data in WU by drop down menus and having to generate your own custom URL to see it, the monthly and yearly totals and max/min should be properly reported where everyone can see without having to do any custom URL trickery.
https://www.wxforum.net/index.php?topic=3752.0