You should use dir2atr on pc and prepare the contents of the .atr file with dir2atr , put a dos in the atr file and the *.bas file which you want to execute in the same atr file. to do that create a directory and put the dos file and the bas file in it. Then execute dir2atr and choose the directory which contains the dos and the bas file. it will convert it to .atr file. then copy that atr file to your sd card and put the sd card in your sio2sd . The trick is , you can not do what you are wanting with all dos versions. you should try one by one , which dos works good for loading .bas files with it. For me mydos 4.53 works good with my unocart. for sio2sd maybe another dos version works good , you can download a huge dos repository from here :
to test if it works :
after booting with sio2sd . type dos
type A to see the contents
if you can see the .bas file , press reset or type B (can not know which one because I dont have a sio2sd) try both of them one by one.
you should get a ready prompt.
type : LOAD "D1:PROGRAM.BAS
note : change the PROGRAM.BAS with the name of the bas file before typing .then type LIST .
if it doesnt list the contents of the .bas file or locks etc.. , then the dos version you used is not compatible. try this until you can see the contents of the bas file.