Many if not most Improvers will have existing portfolios. When customers engage with you, they are very likely to be interested in seeing what jobs you have done before to better assess your level of expertise.
Just because you did a job outside Improved, it doesn't mean it is not yours, so here are the steps you can take to add your previous jobs to your account and portfolio:
1. Create a New Job and provide all the information (photos, description, etc) as if you were the customer
2. Post the Job
3. Select "I'll do it myself" on iOS or "Stop waiting" on Android
4. Add more photos as you see fit
5. Select "Complete job"
Add as many jobs as you think will communicate to your future customers what your capabilities are.