Warning: before you proceed, be sure to backup everything!
Creating a new volume on a existing lvm group is easy.
Check free blocks / space on lvm group
vgdisplay vg1|grep "Free PE" Free PE / Size 29828 / 116.52 GiB
In my case 116 GB were unallocated.
Create the new logical volume
Assume you want to create a new lv “hugo” with 100 GB on lv vg1:
lvcreate -L100G -nhugo vg1
Create a ext4 filesystem on the logical “hugo” volume
Wasn’t that easy :-)?