Hi,

Following the instruction from : 
https://wiki.contextgarden.net/Use_the_fonts_you_want

mtxrun --script font --list --file -pattern=*casual*


gives :

familyname     weight   style    width    variant   fontname 
       filename                                subfont   fontweight

lucidacasual   normal   normal   normal   normal    lucidacasual 
   /home/xxxx/.fonts/LucidaCasual.ttf



Then the following code should work :

\starttypescript [myserif]

   \definefontsynonym[LucidaCasual-Regular]    [file:LucidaCasual.ttf]

\stoptypescript

\starttypescript [myserif]

   \setups[font:fallback:serif]

   \definefontsynonym[Serif] [LucidaCasual-Regular] [features=default]

\stoptypescript

\starttypescript [myserif]

   \definetypeface [myserif] [rm] [serif] [myserif]    [default]

\stoptypescript

\setupbodyfont[12pt]

\starttext

The quick brown fox jumps over the lazy dog

\setupbodyfont[myserif]

The quick brown fox jumps over the lazy dog

\stoptext