Add custom fonts in drop down list of HTML filed

Hello,

•  There is filed (Multiple Line of Text[HTML]) in the list .
• When the user enter text on this filed . There is a drop down List for fonts in "Format Text" tab.
• This drop down has "Tahoma , Georgia , .........." .
• I need to add another custom fonts in this drop down list. how Can I do that?

   Thanks

ASk