We actually put something together recently at Serverless Stack to helping using Serverless Framework and AWS CDK together. Serverless’s strength is working with Lambda and API Gateway. So a good pattern here is to use CDK for the rest of your infrastructure, and Serverless for your Lambda functions.
So we created a tool to do exactly this - Serverless Stack Toolkit (SST), which allows you to combine CDK and Serverless Framework - https://github.com/serverless-stack/serverless-stack
You can deploy your Serverless service and CDK service like sls deploy --stage dev and sst deploy --stage dev.