Is it be possible to upgrade Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 to Oracle Database 11g Enterprise Edition Release 18.104.22.168.0 using the same oracle home for 11g i.e (/u01/app/oracle/product/11.2.0/dbhome_1) during the DBUA process
Yes, it is possible, you can have one engine and n databases. You differences between databases by SID.
HTH - Antonio NAVARRO.
Yes - pl see the Upgrade Guide - http://docs.oracle.com/cd/E11882_01/server.112/e23633/toc.htm
You can skip the portion of the doc where it states to install 11gR2, since you already have the binaries installed.