One way would be to set your ClearCase preferences to unreserved by default.
See "To set the default for reserved or unreserved checkouts (Windows)e"
Tools > Options
and, in the Options window, click ClearCase
Options.
Click Start > Programs > IBM Rational > IBM Rational ClearCase > User Preferences
.
In the ClearCase User Options window, in the Check Out field, to make unreserved checkouts the default setting, clear Reserved.
But if you don't want to change the default (or you are on Linux), you can also define a post-op checkout trigger.
cleartool mktrtype -element -all -post checkout -exec '/opt/rational/clearcase/bin/cleartool unreserve -nc '$CLEARCASE_PN' TRIGGERNAME
You would set that trigger just for the duration of the clearfsimport
.