You can create or modify the equalizer presets in the .rockbox/eqs folder with the text editor of your choice.
I did this recently to port an EQ I was testing to rockbox:
Rockbox:
eq enabled: on
eq precut: 66
eq low shelf filter: 105, 7, 90
eq peak filter 1: 8983, 26, 44
eq peak filter 2: 54, 4, -54
eq peak filter 3: 1148, 23, -15
eq peak filter 4: 491, 12, 6
eq peak filter 6: 7451, 39, 20
eq peak filter 7: 4502, 60, -17
eq peak filter 8: 1937, 39, 9
eq high shelf filter: 10000, 7, -39
Qudelix:
CH: 0
TYPE: PEQ
Preamp: -6.6 dB
Xfeed: 1 0
Filter 1: ON LS Fc 105 Hz Gain 9.0 dB Q 0.700
Filter 2: ON PK Fc 8983 Hz Gain 4.4 dB Q 2.630
Filter 3: ON PK Fc 54 Hz Gain -5.4 dB Q 0.480
Filter 4: ON PK Fc 1148 Hz Gain -1.5 dB Q 2.380
Filter 5: ON PK Fc 491 Hz Gain 0.6 dB Q 1.230
Filter 6: ON HS Fc 10000 Hz Gain -3.9 dB Q 0.700
Filter 7: ON PK Fc 7451 Hz Gain 2.0 dB Q 3.910
Filter 8: ON PK Fc 4502 Hz Gain -1.7 dB Q 6.000
Filter 9: ON PK Fc 1937 Hz Gain 0.9 dB Q 3.930
Filter 10: ON PK Fc 5639 Hz Gain -0.9 dB Q 6.000
Filter 11: OFF PK Fc 0 Hz Gain 0.0 dB Q 0.000
Filter 12: OFF PK Fc 0 Hz Gain 0.0 dB Q 0.000
Filter 13: OFF PK Fc 0 Hz Gain 0.0 dB Q 0.000
Filter 14: OFF PK Fc 0 Hz Gain 0.0 dB Q 0.000
Filter 15: OFF PK Fc 0 Hz Gain 0.0 dB Q 0.000
Filter 16: OFF PK Fc 0 Hz Gain 0.0 dB Q 0.000
Filter 17: OFF PK Fc 0 Hz Gain 0.0 dB Q 0.000
Filter 18: OFF PK Fc 0 Hz Gain 0.0 dB Q 0.000
Filter 19: OFF PK Fc 0 Hz Gain 0.0 dB Q 0.000
Filter 20: OFF PK Fc 0 Hz Gain 0.0 dB Q 0.000