No sorry,the only way you could do this would be to get in touch with the people who make the fire and ask them.
You could change the pattern of the flames and cause problems
Best advice as DDAY mentions is to consult you're manual. If you can then the matrix will have to be changed aswell to house the new shape of the pebbles without obstructing the burner ports. If you do not have the manual look for the Make and model of the fire on the data lable on the base of the fire under the burner and contact the manufacture or post it back on here I may have some details about you're fire in my files/data base...
Highly unlikely,the normal "coals" are designed to be heated to glow like real coal,the pebbles are usually bathed in coloured light without being heated and are not designed to be heated in the gas flame
We have a gas fire which has the pebbles on, but it was also supplied with coals too, so we could have either. However as everyone else states check with the fire manufacturer I am sure they would be happy to help.
if you take the coals off to clean or re-paint them you should always replace them back onto the fire in the exact positions specified by the manufacturer in the instruction manual.
you never add extra coals or pebbles.
if you want to replace them completely you must buy the correct coals for the fire you have.
this is vital for your safety as incorrect coals or placing of them can cause lack of aeration to the burner and thus cause the fire to produce soot/carbon monoxide.