Can an FX unit of Traktor be controlled like Slide FX of rekordbox dj?

Yes. The following shows the example of assigning the MIDI of Slide FX on the left side of the unit to FX unit 1 of Traktor.

1. Open the [Preferences] window in Traktor and select [Controller Manager].

2. Set Device to [Generic MIDI] and set In-Port and Out-Port to [DDJ-XP1].

3. Click [Add In...], select [FX Unit] > [Knob 1] from the pull-down menu, and set Device Mapping to [Ch05.CC.002].

4. Set Type of Controller to [Fader/Knob], Interaction Mode to [Direct], and Assignment to [FX Unit 1].

5. Click [Add In...], select [FX Unit] > [Knob 2] from the pull-down menu, and set Device Mapping to [Ch05.CC.003].

6. Set Type of Controller to [Fader/Knob], Interaction Mode to [Direct], and Assignment to [FX Unit 1].

7. Click [Add In...], select [FX Unit] > [Knob 3] from the pull-down menu, and set Device Mapping to [Ch05.CC.004].

8. Set Type of Controller to [Fader/Knob], Interaction Mode to [Direct], and Assignment to [FX Unit 1].

9. Click [Add In...], select [FX Unit] > [Button 1] from the pull-down menu, and set Device Mapping to [Ch05.Note.B4].

10. Set Type of Controller to [Button], Interaction Mode to [Toggle], and Assignment to [FX Unit 1].

11. Click [Add In...], select [FX Unit] > [Button 2] from the pull-down menu, and set Device Mapping to [Ch05.Note.C5].

12. Set Type of Controller to [Button], Interaction Mode to [Toggle], and Assignment to [FX Unit 1].

13. Click [Add In...], select [FX Unit] > [Button 3] from the pull-down menu, and set Device Mapping to [Ch05.Note.C#5].

14. Set Type of Controller to [Button], Interaction Mode to [Toggle], and Assignment to [FX Unit 1].