You can refer below link for "memory module installation guidelines". As you are mixing of memory with different speed it will work only with lowest speed.  [URL]    Can you share the part number of current DIMM and new DIMM? 
Greetings! Existing the Server have 10 Ram with 32G 2400Mhz, and I going to increase 8 more Ram with 32G 2933Mhz.  Is that compatible ? I'm more worry about speed different cause to any error in future 
Ram Upgrade for Server R640