I did it this way (drive j: was available)

net use j: \\wsl$\Ubuntu

Note that this is persistent, and if not deleted can cause wsl2 to restart by itself. After doing this I was able to access files on the Ubuntu distro from windows emacs.

to delete it:

net use /del j:

As answered here, the command-line tool

subst

Associates a path with a drive letter.

SUBST [drive1: [drive2:]path]
SUBST drive1: /D

  drive1:        Specifies a virtual drive to which you want to assign a path.
  [drive2:]path  Specifies a physical drive and path you want to assign to
                 a virtual drive.
  /D             Deletes a substituted (virtual) drive.

Type SUBST with no parameters to display a list of current virtual drives.

Source: A very similar question/answer

In my specific case;

• I first made visible to WSL2 a USB attached Harddrive following these instructions;

1. Listing the available disks in Windows

• PowerShell: GET-CimInstance -query "SELECT * from Win32_DiskDrive"
• Command Prompt:wmic diskdrive list brief

.

> wmic diskdrive list brief
Caption                                    DeviceID            Model                                      Partitions  Size
SK hynix PC801 HFS001TEJ9X101N             \\.\PHYSICALDRIVE0  SK hynix PC801 HFS001TEJ9X101N             3           1234567890120
Samsung SSD 840 PRO Seri SCSI Disk Device  \\.\PHYSICALDRIVE1  Samsung SSD 840 PRO Seri SCSI Disk Device  6           256128643216

2. Mounting the drive to expose it to WSL2

.

> wsl --mount \\.\PHYSICALDRIVE1 --bare
The operation completed successfully.

3. From with in WSL2 : List the partitions and their types and mount it from within WSL

.

~~~@~~~~~:/$ lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
sda      8:0    0 388.4M  1 disk
sdb      8:16   0     4G  0 disk [SWAP]
sdc      8:32   0     1T  0 disk /snap
                                 /mnt/wslg/distro
                                 /
sdd      8:48   0 238.5G  0 disk
├─sdd1   8:49   0   499M  0 part
├─sdd2   8:50   0   300M  0 part
├─sdd3   8:51   0   128M  0 part
├─sdd4   8:52   0 190.1G  0 part
├─sdd5   8:53   0    31G  0 part
├─sdd6   8:54   0  15.4G  0 part
└─sdd7   8:55   0  1024M  0 part
~~~@~~~~~:/$ sudo blkid -s TYPE /dev/sdd4
/dev/sdd4: TYPE="ntfs"
~~~@~~~~~:/$ sudo blkid -s TYPE /dev/sdd5
/dev/sdd5: TYPE="ext4"
~~~@~~~~~:/$ ls /mnt/
c  d  wsl  wslg
~~~@~~~~~:/$ sudo mkdir /mnt/g
~~~@~~~~~:/$ sudo mount -t ext4 /dev/sdd5 /mnt/g

4. Navigate on Windows explorer to the folder of interest and give it a Virtual Drive name with command subst in an Admin Command Prompt:

• > subst g: \\wsl.localhost\Ubuntu\mnt\g\