The list of D-Star Repeaters and contained in host files. Typically they are DCS_Hosts.txt DPlus_Hosts.txt and DExtra_Hosts.txt. You can modify any of these to include the reflector that you want to reach. The DExtra_Host.txt file is where you'll add XLX reflectors but they will be called XRF in the hosts file instead of XLX. Whenever possible use the ip address of the reflector being added.
Pay special attention to the format. These are treated as data files and they use delimiters to separate the data on a line. Typically they well be tab delimited but I've seen them in comma and semi-colon also.
Regardless, follow the format of the existing file and you'll usually have success.
The files are often in different locations so you may have to hunt for them. In ircDDBGateway they are in /usr/share/ircddbgateway/ if you performed a make install. Pi-Star has it's own instructions, openSpot has a different method, etc.