can I import fonts into LibreOffice

I want to import Celticmd

Is it a TrueType-Fonts? TrueType - Wikipedia

Fonts are usually provided via the OS.

You can’t “import” fonts in Libre. You can download fonts from anywhere and install them, after which they are available in all programs that allow the font type.

Cool font.
Fonts are installed in your operating system.
Then the fonts are available to all applications - including LibreOffice.