Yes, you can reate and upload your custom controls to Sitefinity without any problems, actually we have dedicated a tutorials section on creating custom controls
in our Online Documentation, I believe you will find it very useful. Please do not hesitate to let us know if you have any additional questions, we'll be glad to help you further.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items