How to create a new _time field by merging two separate fields

[dailv1808]

I have a sample event log below:

2018-05-04 06:59:50 AAA="1", BBB="2", CCC="3", XXX="70029", ZZZ="2018-05-04 00:00:00"

Splunk will automatically get "2018-05-04 06:59:50" for _time field. But i want the _time field will be created by XXX and ZZZ fields.
like this: _time = "2018-05-04 7:00:29"
Note that I haven't permission to access to props.conf file. So how can i do it.

Any suggestion is welcomed!
Thank for advance!

[HiroshiSatoh]

You can set _time in the search sentence.

| makeresults |eval  XXX="70029", ZZZ="2018-05-04 00:00:00"
| eval bk_time=_time
| eval _time=strptime(substr(ZZZ,0,10)+":"+substr("0"+XXX,len("0"+XXX)-5,6),"%Y-%m-%d:%H%M%S")