If the custom field is associated with the respective users' AD account object then you should be able to map it directly in SharePoint 2013's User Profile service similar to how it would have been created for your SP2010 environment - https://technet.microsoft.com/en-us/library/ee721049.aspx#MapUserProc
If the custom field only exists as a SharePoint field/property in SP2010 the above would still mostly be true, though a separate mechanism would need to be in place for the field to be populated.
Which method are you using to move from SP2010 to SP2013?