Slopes is able to import recordings from other ski and snowboard and GPS hardware that exprots to GPX (such as the Suunto 9 Baro, Garmin fēnix 5S Plus, Polar Vantage M, and others). So even if you didn't know about Slopes at the time, it is never too late.
Don't worry, the days you import via GPX get all of the features that makes Slopes great. So you won't miss out on the in-depth stat breakdowns, competing with your friends on the seasonal leaderboards, the detailed timeline view, or anything else.
Import your activities via GPX to Slopes and have all your epic winters in one place!
This step depends a bit on your device's brand. For example how to get GPX files from Garmin devices can be found here. Usually pretty easy to google "device name GPX export" for a guide. GPX is a very common GPS file format, so most major devices support exporting to it.
Note that the exported file needs the following data to be included: latitude, longitude, altitude, and time. Data accuracy is improved if the file also contains vdop and hdop (vertical and horizontal accuracy) data.
Move these GPX files to your phone. Use your favorite file transfer service like Dropbox, on a Mac you can use AirDrop (if you have more than one file you can AirDrop them all at once) or iCloud Drive, with an Android use your SD card, or there is always the good old fashioned email it to yourself trick.
If you're using Slopes on iOS, Slopes does support iTunes File Sharing so you can bulk import these files and skip the manual importing.
Open Slopes, head to the Logbook, and hit the + in the top right to start the import. Head to where you copied the files on your phone. Pick the GPX file that you would like to import. (On Android you can also long-press to select and import multiple files at the same time.)
If you emailed the files to yourself, or were using another third party app to move the files over, you'll either want to tap the attachment and find a way to open the file with Slopes, or save them off to your phone (like the downloads folder) and then import via the logbook.
And that's it! Slopes will read and import the days pretty quickly.