This Geekyfacts article explains the steps to prepare Solaris x86 miniroot with additional device driver,                               

1) Copy the x86.miniroot from iso image or from jumpstart stage area to temp location

[root@geekyfacts]#cp /jumpstart-location/boot/x86.miniroot /tmp/x86.miniroot

2)Unpack the miniroot in /var/tmp/miniroot

[root@geekyfacts]#/boot/solaris/bin/root_archive  unpack /tmp/x86.miniroot  /var/tmp/miniroot

3) Install the required device drivers in /var/tmp/miniroot

[root@geekyfacts]#pkgadd -R /var/tmp/miniroot -d

4) Packing the miniroot after installing driver

[root@geekyfacts]#/boot/solaris/bin/root_archive   pack /var/tmp/x86.miniroot /var/tmp/miniroot

5) Copy back the new miniroot to original location

[root@geekyfacts]# mv /jumpstart-location/boot/x86.miniroot /jumpstart-local/boot/x86.miniroot.old

[root@geekyfacts]#cp /var/tmp/x86.miniroot  /jumpstart-location/boot/x86.miniroot