To answer your question, PaaS is simply another way of saying that an IIS site exists that allows you to run a website. Said differently, using the Sitecore Azure Toolkit to deploy a vanilla instance of Sitecore 9.0.2 to Azure PaaS and then deploying your code base through a WebDeploy package should be sufficient to utilize PaaS.
Things to consider when moving to PaaS:
- Are you using Solr or Azure Search?
- How will you migrate your content? I would advocate that if you are using Unicorn/TDS, let it deploy out the item structure first, and then package up any content from the On Prem site and install on the PaaS Sitecore.
- Ensure that your configuration transforms are correctly done taking into account the domain name changes for most of xConnect.
- If you aren't already, consider using Azure DevOps for Deployment (formerly VSTS).
- Ensure that Session State is not InProc and that you are using Azure Redis Cache for Sessioning.
I think a larger conversation is questioning the intention around using WebForms and not MVC, but I don't want to detract from the question at hand.