This windows 64bit executable converts the GIB-UVA dataset (A BCI P300 Speller) into NPX file format. You can access the dataset here: 

GIB-UVa ERP-BCI dataset | IEEE DataPort (ieee-dataport.org)

This Matlab code converts Akimpech (A BCI P300 Speller) into NPX file format. You can access the dataset here: 

P300 Akimpech database (uam.mx)

This Matlab code converts BCI Competition III - DataSet II (BCI P300 Speller) into NPX file format. You can access the dataset here: 

BCI Competition III (bbci.de)

This Matlab code converts BCI Competition II - DataSet IIb (BCI P300 Speller) into NPX file format. You can access the dataset here: 

BCI Competition II (bbci.de)

This Matlab code converts BNCI DataSet 9 (A BCI P300 Speller) into NPX file format. You can access the dataset here: 

Data sets - BNCI Horizon 2020 (bnci-horizon-2020.eu)

This Matlab code converts BNCI DataSet 8 (A BCI P300 Speller) into NPX file format. You can access the dataset here: 

Data sets - BNCI Horizon 2020 (bnci-horizon-2020.eu)