im not a big fan of the look of the sillica sand, but i imagine it could be done.
as far as adding sand, just realize that if you add a finer sand to a coarser sand, the finer will always settle to the bottom...but you can definately add stuff, but make sure to do it very slowly. add liek a pound of sand a bit at a time for a week., untill you get the desired look you want!
good luck
jon