XPS 8930, add Crucial Pro 32GB kit?

If you purchase the memory directly from Crucial, they will provide support and guarantee the compatibility with free return  https://www.crucial.com/compatible-upgrade-for/dell/xps-8930

You may try to get 3x 8GB module to combine with your existing 8GB for a total of 32GB configurations

Or, you can get 4x 16GB for a total of 64GB configurations.

Or, you can take a chance with your planned purchase of 32GB kit (2x 16GB) however, it is not a supported memory configuration.  Below is the chart of memory configurations matrix with DIMM locations

Besides Dell ram matrix kindly provided by chino,  most people would install the 2x16 GB in the two white dimm slots. Although this is not shown in the matrix, it is traditionally the most common way of having 2 matched ram sticks.

 The reason why Dell did not have it in matrix is probably b/c they did not validate that configuration using their oem ram.  That is not an exclusive configuration, just you know.

having 2x16 + 1x8 would not be a great idea let alone not certain it would post. 2x16 allow you dual channel bandwidth.  An unmatched 1x8 throws in a monkey wrench.  You could add 2x8 matched ram in two black slots to maintain dual channel and accomplish 48 GB total

Hope it works for you. I can tell you for me the very same upgrade failed for me twice. 1st I got 2 Micron installed in black dim slots, got Amber blinking power light. Verified installed correctly multiple times.  Returned cards. Then purchased Crucial and same thing. So still at 16GB.