You need to edit the OCA rules, which are stored in
<install_dir>/share/esopipes/xshoo-<version>/reflex/xsh_wkf.oca
(or if you want to process all data in STARE mode edit
<install_dir>/share/esopipes/xshoo-<version>/reflex/xsh_wkf_stare.oca).
There you need to look for SLIT data for the lines
select execute(ACTION_SCI_SLIT_STARE_UVB) from inputFiles where RAW.TYPE=="SCI_SLIT_STARE_UVB"
group by DET.READ.CLOCK,SEQ.ARM,INS.OPTI3.NAME,INS.OPTI2.NAME,ARCFILE;
select execute(ACTION_SCI_SLIT_STARE_VIS) from inputFiles where RAW.TYPE=="SCI_SLIT_STARE_VIS"
group by DET.READ.CLOCK,SEQ.ARM,INS.OPTI4.NAME,INS.OPTI2.NAME,ARCFILE;
select execute(ACTION_SCI_SLIT_STARE_NIR) from inputFiles where RAW.TYPE=="SCI_SLIT_STARE_NIR"
group by DET.DIT,SEQ.ARM,INS.OPTI5.NAME,INS.OPTI2.NAME,ARCFILE;
select execute(ACTION_SCI_SLIT_STARE_JH_NIR) from inputFiles where RAW.TYPE=="SCI_SLIT_STARE_JH_NIR"
group by DET.DIT,SEQ.ARM,INS.OPTI5.NAME,INS.OPTI2.NAME,ARCFILE;
and for IFU data for the lines
select execute(ACTION_SCI_IFU_STARE_UVB) from inputFiles where RAW.TYPE=="SCI_IFU_STARE_UVB"
group by DET.READ.CLOCK,INS.OPTI2.NAME,SEQ.ARM,ARCFILE;
select execute(ACTION_SCI_IFU_STARE_VIS) from inputFiles where RAW.TYPE=="SCI_IFU_STARE_VIS"
group by DET.READ.CLOCK,INS.OPTI2.NAME,SEQ.ARM,ARCFILE;
select execute(ACTION_SCI_IFU_STARE_NIR) from inputFiles where RAW.TYPE=="SCI_IFU_STARE_NIR"
group by DET.DIT,INS.OPTI2.NAME,SEQ.ARM,ARCFILE;
and replace
ARCFILE
by
TPL.START as (TPL_A,tpl)
so that
select execute(ACTION_SCI_SLIT_STARE_UVB) from inputFiles where RAW.TYPE=="SCI_SLIT_STARE_UVB"
group by DET.READ.CLOCK,SEQ.ARM,INS.OPTI3.NAME,INS.OPTI2.NAME,ARCFILE;
changes to
select execute(ACTION_SCI_SLIT_STARE_UVB) from inputFiles where RAW.TYPE=="SCI_SLIT_STARE_UVB"
group by DET.READ.CLOCK,SEQ.ARM,INS.OPTI3.NAME,INS.OPTI2.NAME,TPL.START as (TPL_A,tpl);

In this way the pipeline will merge the multiple exposures taken in a single template and will process the combined frame. If instead you want to stack all data obtained within one OB replace TPL.START by OBS.START. We recommend to apply the modification in a new oca file, named e.g. xsh_wkf_modif.oca and then to load this file into the DataOrganiser.