Yes, there are the string justification methods, ljust and rjust.

>>> '12'.rjust(5, '#')
'###12'
>>> 'txt'.rjust(5, ' ')
'  txt'
>>> '12'.ljust(5, '#')
'12###'

Looking at Text | Text functions | ArcGIS Arcade | ArcGIS Developers

Text

Text(value, format?) -> Text

The format parameter is where you can specify the number of digits. For example:

var num = 12004;
return Text(num, '000000');

yields:

012004

If you want to make it dynamic, it is a bit cumbersome but doable:

var num = 12004;
var pad_len = 6;
var pad_char = 0;
return Text(num,  Concatenate(array(pad_len,pad_char)));

yields:

012004

As maybe a alternative more portable [1] and efficient [2], actually you can just use str.ljust.

In [2]: '190'.ljust(8, '0')
Out[2]: '19000000'

In [3]: str.ljust?
Docstring:
S.ljust(width[, fillchar]) -> str

Return S left-justified in a Unicode string of length width. Padding is
done using the specified fill character (default is a space).
Type:      method_descriptor

[1] format is not present on old python versions. format specifier was added since Python 3.0 (see PEP 3101) and Python 2.6.

[2] reverse twice is an expensive operation.

To pad strings:

>>> print('4'.zfill(3))
004
>>> print('-4'.zfill(3))
-04

To pad numbers:

>>> n = 4
>>> print(f'{n:03}') # Preferred method, python >= 3.6
004
>>> print('%03d' % n)
004
>>> print(format(n, '03')) # python >= 2.6
004
>>> print('{0:03d}'.format(n))  # python >= 2.6 + python 3
004
>>> print('{foo:03d}'.format(foo=n))  # python >= 2.6 + python 3
004
>>> print('{:03d}'.format(n))  # python >= 2.7 + python3
004

String formatting documentation.

there is no inbuilt zfill but I use this zfl function

def zfl(s, width):
# Pads the provided string with leading 0's to suit the specified 'chrs' length
# Force # characters, fill with leading 0's
return '{:0>{w}}'.format(s, w=width)

This might be useful to you? Just pass the string and the string width you require.