First sentence in "drum part" section of the reference manual:

Each Drum Part can consist of up to 73 Drum Keys, assigned to notes spread across the keyboard (C0 to C6).

All drum parts have this direct trigger limitation.

If you want A-1 to trigger a sample, you have these options:

1) Keep using drum PART but assign an ARP to press a fixed note between C0 and C6. The ARP can be triggered by a note outside the range of C0-C6.
2) Use an AWM2 PART instead. This becomes more difficult to control since a key needs to be held down to allow the sample (Waveform) to continue playback. You can use an arpeggio to solve this. However, if you are using an ARP - you might as well pick option #1 as it is overall more simplistic. Or you can hold down the key in order to play the sample. AWM2 can have a sample mapped to any MIDI note.
3) In the land of the strange - you can do something not-so-suggested. It should work, although is not perhaps the best routing practice. You can setup an AWM2 Part with Part Zone settings (and master zone on) that alters the output octave vs. the key you press on the keyboard (Octave Shift in zone control). Turn Int SW off if this Part is only used to handle the octave shift. Then, when you press A-1, the output of this Part will transmit a higher MIDI note. You can also set the transmit MIDI channel to say Part 9 (or any unused Part with keyboard control turned off). You can place your Drum Part in Part 9 (or the Part matching your zone transmit) and place your desired sample ("sound" ) into the "A" that matches the octave shifted by the other Part setup for transmit. With your keyboard setup for MIDI IN/OUT as "MIDI" (not "USB" ) then you can connect a standard MIDI cable between MIDI IN and MIDI OUT ports. With all of this setup, the Zone output Part will target the Part with your drum part and will handle making the octave shift for you. You can even use Zone transpose, if you want, to map A-1 to a different MIDI note (not just an "A" - but any note). That's up to you. I tested this and it all works.

I have a MODX7 with a 88-keys midi controller connected.
I want to assign a sound to the lowest key on the midi-controller (key: A-1) but the MODX will only let me go to C0.
Is there no way to assign sounds to the three lowest keys on the midi-controller?

On the MODX, *select* the Part with your Drum Kit.
Press [EDIT]
“Part x”, “Common” appears in the lower left corner

Tap “Part Settings” > “General”
Find “NOTE SHIFT”
SET “Note Shift” = +3

STORE

C0 now equals A-1 on your 88 key controller
C#0 now equals A#-1 on your 88 key controller
D0 now equals B-1 on your 88 key controller