if you would like to have spaces in name, you can use normal physical model and insert this value to Caption and in Name you will store physical name without spaces. So first step for you is convert your logical model to SQL Server 2014 or other db oriented model. You get physical model, but still with spaces in names.
Second step is define Naming Convention. Please Open Model Properties Dialog, click on tab Naming Convention and choose Manage Naming Convention.
Create New Naming Convention and Edit it.
Go to Glossary tab, and define character replacement set. In this set we just replace space with underscore.
Go to Naming Convention Rules tab and select Entities, Attributes, or other objects. All Apply and assign this Naming Convention to model.
From this point when you add new attribute and you will insert caption with spaces it will be automatically change to underscores in Name. For Already existed spaces in Name please go to main menu-Tools-Naming Convention-Verification...
Just select what you want to change (probably all) and press update.