Should be very simple to do. To get SIM contacts into the phone memory, export the SIM contacts as a .csv file and import that file into the Contacts app.
The option to save contacts to SIM cards is disappearing and no doubt will be gone completely in a year or two. Manufacturers have already agreed that they will work together to eliminate SIM cards altogether. Unlike other contacts options, SIM contacts can have names and numbers only - no email addresses, physical addresses, photos, birthdays, notes - none of the extra information most people like to be able to add. In short, SIM contacts is old, outdated technology that should not be considered a viable option.
All that said, I very strongly recommend saving contacts to your Google account. This provides instant online backup of your contacts making it nearly impossible to ever lose them. Also makes reloading or transferring contacts to a new device a snap. And they can automatically sync between multiple devices, computers, etc.