You can import contacts into FrontlineSMS as a .csv file (comma separated value).
If you can export contacts as a .csv from a program you are currently using, then all you need to do is adjust the column headings. The column headings should follow the example in the screenshot below. Note that "group" is optional; it will be interpreted by FrontlineSMS as a group tag for the contact.
If you need help getting the format correct, you could enter a few contacts directly into FrontlineSMS, then export those from the software as a .csv file. Open this file in a spreadsheet application such as OpenOffice or Microsoft Excel, and add your contacts under the appropriate columns. You can probably copy and paste these from your own database. Once finished, save your new contacts file as a .csv, and import into FrontlineSMS.