The directions over at the SBBS wiki didn’t seem to work for me so I did some hunting and found the following instructions which allowed it to work using Netfoss:

Synchronet BBS 3.1x USAGE

Synchronet already has its own FOSSIL support, but using NetFoss in place
of the internal FOSSIL can allow DOS doors to run considerably faster,
often by a factor of 2 or more times faster then the internal speed, with
lower CPU usage. You can use NetFoss to run all or just some or all of
your door programs, and run others using the internal FOSSIL.

Synchronet can create a DOOR32.SYS file, but we do not suggest running
NetFoss in DOOR32.SYS mode because Synchronet is unable to create both a
DOOR32.SYS and a standard drop file at the same time. For this reason the
DOOR32.SYS mode should not be used at the time this guide was written.

Here is how to configure the “Legend Of The Red Dragon” door in Synchronet
3.10j using the Non-DOOR32.SYS mode:

Internal Code LORD
Start-up Directory C:SBBSXTRNLORD
Command Line c:sbbsnf.bat /N%# /H%H start.bat %#
Clean-up Command Line
Execution Cost None
Access Requirements
Execution Requirements
Multiple Concurrent Users Yes
Intercept Standard I/O No
Native (32-bit) Executable Yes
Use Shell to Execute No
Modify User Data No
Execute on Event No
Pause After Execution No
Place Drop File In Node Directory
Time Options…

Notice that the Native (32-Bit) Executable option is enabled. This needs
to be turned on in order for Synchronet to not enable its own internal
FOSSIL driver. REPEAT – even though you are not using DOOR32.SYS as your
dropfile, Native (32-Bit) Executable must be enabled. Additionally, make
sure to change the command line to reflect the directory that you
installed NetFoss and the Start-up directory should either reflect where
your door is located if you don’t use a batch file to start the door, or
could have the startup directory point to your current node directory
where the dropfiles are created. (If you do the later, you should launch
the door with a batch file that first uses the CD command to Change the
Directory to where the door is located.

When using the Non-DOOR32.SYS mode, you must edit your NF.BAT file to add
the ” %1″ at the end of the second line, as explained earlier in this
document. Instructions can also be found in the NF.BAT.

Make sure to change the Command line in NF.BAT to reflect the directory
that you installed NetFoss in, and the Start-up directory should reflect
where your door is installed.

In the LORD door example above, the start.bat is the batch file located
in the Start-up Directory which actually runs this door game.