how we can insert casefeed of one case to another case through coding???

ParentId on CaseFeed is not writable.

If you look at the CaseFeed attributes, you can find that it is queryable: true but not createable: false, updateable: false etc.

Attributes are:
activateable: false
compactLayoutable: false
createable: false
custom: false
customSetting: false
deletable: true
deprecatedAndHidden: false
feedEnabled: false
hasSubtypes: false
idEnabled: true
isSubtype: false
keyPrefix:
label: Case Feed
labelPlural: Case Feed
layoutable: false
mergeable: false
mruEnabled: false
name: CaseFeed
networkScopeFieldName:
queryable: true
replicateable: true
retrieveable: true
searchable: false
undeletable: false
updateable: false