If you have created workflow in project server 2010 using Solution Starter and wondering if you could migrate them to Project Server 2013 then answer is NO.
All your stages, EPT’s, PDP’s and Phases will get migrated to Project Server 2013, however the workflow won’t get migrated. Also you cannot use Solution Starter to re-create workflow in Project Server 2013.
You need follow below steps to get your workflow working in Project Server 2013.
1) Re-create workflow in Project Server 2013 using SharePoint Designer 2013. This tool can be freely downloaded and will help to create a workflow without any code. To create workflow in Project Server 2013 you need to setup and configure the workflow manager http://technet.microsoft.com/en-us/library/dn458867(v=office.15).aspx and need to make sure you have the SharePoint Designer 2013 installed on your client system to create workflow http://www.microsoft.com/en-in/download/details.aspx?id=35491
2) Attach the newly created workflow to your EPT.
3) For all in-process workflow you need to restart the workflow using below steps.
a) Server Settings >> Select Change or restart workflow
b) Select the list of in-process projects >> Restart the workflow and select either Skip until the current workflow stage or Skip to a particular workflow stage (This assumes that the current/selected workflow stage is available in the target workflow. If not, the workflow will not skip)