Re: How to use Existing Sequential Visual studio 2010 workflow in Sharepoint Online ?

Hi PointDeveloper,
 
Currently, Office 365 SharePoint Online supports custom solutions. Some farm solutions are supported on SharePoint On-premise, however, it's not necessary to be supported on SharePoint Online.
 
Moreover, considering the workflow referring to custom solution, we have a group for dedicating to deal with the kind of issue. I recommend you post the question to the MSDN forum.

Additionally, I will provide some articles below for your reference,including the features available to developers for Office 365. 
SharePoint Online Quick Links
SharePoint Online for Office 365 Developer Guide 
SharePoint Online Developer Resource Center

Thanks,
Ida Qiu