in spfile:
the name is 'event' and values are quoted then comma separated as in:
ALTER SYSTEM SET event='10235 trace name context forever, level 2','27072 trace name errorstack level 3' COMMENT='TEST' SCOPE=SPFILE;

If bad syntax, you will get ORA-2194 at next database restart. so double check and copy old spfile just in case.

You can remove all events with:
ALTER SYSTEM RESET EVENT SCOPE=SPFILE SID='*' ;

in memory:
the name is 'events' and values are column-separated then quoted as in:
ALTER SYSTEM SET events='10235 trace name context forever, level 2:27072 trace name errorstack level 3';

in pfile:

- the name is 'event' and values are column-separated then quoted as in:
event='10235 trace name context forever, level 2:27072 trace name errorstack level 3'

- or set parameters one by one (must follow eachother):
event='10235 trace name context forever, level 2'
event='27072 trace name errorstack level 3'

Finally here is the event syntax (from http://www.larsditzel.de/9iquickref.pdf)

{ alter system set event = | alter session set events [=] } ‘ trace name context {forever, level | off}’

alter session set events [=] {
‘immediate trace name { heapdump | blockdump | treedump | controlf | systemstate | buffers } level ’
| ‘ trace name errorstack level [; name processstate level ]’
}

{ alter system set event = | alter session set events [=] } ‘ trace name context {forever, level | off}’

alter session set events [=] {
‘immediate trace name { heapdump | blockdump | treedump | controlf | systemstate | buffers } level ’
| ‘ trace name errorstack level [; name processstate level ]’
}