Export your WhatsApp messages and convert them to Signal messages.
Short introduction to the code:
You'll need both your Android smartphone and a desktop with a Python terminal (I prefer Spyder but any one of them should work). If you have both of these, you should be able to follow the steps as indicated in the Python document (ConvertWhatsAppToSignal.py). To run this in you Python terminal, you open the terminal and go to (File -> Open...) and select ConvertWhatsAppToSignal.py that you downloaded to your pc.
The idea is that you'll export your WhatsApp conversations to backup-files on your smartphone. Next you send these files to your desktop and put them all together in a folder. Then you can use the software to transform these files into SMSBackupAndRestore-files so that when you place these files back on your smartphone, SMSBackUpAndRestore will be able to import the files into your system-sms database. Lastly, you can use Signal's built-in tool to import your system-sms database into Signal's app. Now you should have all your WhatsApp messages in Signal.
This process is explained step-by-step in the Python document, so take a look at it before starting.
If you like to help us in getting Signal to implement it in their app, please upvote/retweet: Reddit post and Twitter post
For Germans:
WhatsApp doesn't allow exporting chats in Germany so this is the next best thing as far as I know. If you just want to save your chats and not necessarily export them to Signal however, then you can go to web.whatsapp.com, pick a conversation, scroll all the way to the top of the conversation and then right-click anywhere but the conversation and choose "save as...". The result of saving this is a webpage and a folder with the contents of that webpage. This will allow you to save your conversations as webpages on your desktop (it looks like a webpage when you open it, but it isn't actually on the internet, everything is stored on your computer). You'll need to do this separately for each conversation so it might be a bit time consuming if you have some large conversations that you'll have to scroll through (remember, you can use scroll-lock by clicking your scroll wheel and moving your mouse in the direction you want to scroll).